Cosmos 2139 was a Russian Global Navigation Satellite System (GLONASS) satellite launched from the Baikonur cosmodrome aboard a Proton rocket. Originally established in order to locate the Soviet Union's civil aircraft and its merchant and fishing vessels, the signals were used by many American GPS system receivers as a complement/backup to the GPS system itself. The operational system contained 21 satellites in 3 orbital planes, with 3 on-orbit backups.